SIOUX FALLS, S.D. - A conservation group has appealed a judge's dismissal of its lawsuit that alleges the coal-fired Big Stone power plant in northeast South Dakota violated federal environmental laws by not installing modern pollution controls.

The Sierra Club complaint asked U.S. District Judge Charles Kornmann of Aberdeen to order the owners of the plant near Milbank to install equipment that would reduce pollution before they build a second plant on the site, which the group also opposes.

Big Stone is owned by Otter Tail Power Co., Montana-Dakota Utilities Resources Group Inc. and NorthWestern Corp.

Sierra Club said the utilities should not be allowed to build a second electricity-generating plant at the site when they have not installed modern pollution controls on the existing plant.

The lawsuit alleges that federal law required Big Stone to install new pollution equipment when it made three changes in the past decade: a switch to coal from Wyoming's Powder River Basin in 1995, the replacement of a heater in a boiler in 1998 and changes made to supply steam to an ethanol plant in 2001.

The utilities asked that the lawsuit be dismissed, largely because the claims were not filed within the five-year statute of limitations and aren't allowed on a valid Clean Air Act permit issued by the state of South Dakota.

Kornmann agreed, dismissed the lawsuit and refused to reconsider the case. The Sierra Club took it to the 8th U.S. Circuit Court of Appeals late last week.

Lawyers for both the Sierra Club and Big Stone declined to comment publicly on Tuesday.

It and other environmental organizations also oppose Big Stone II and the construction of transmission lines to carry the additional power into Minnesota.

Five power companies will decide this fall whether to proceed on the proposed $1.6 billion coal-fired power plant that would serve more than 1 million customers in Minnesota, the Dakotas, Montana and Iowa. Besides Otter Tail, other partners are Central Minnesota Municipal Power Agency, Heartland Consumers Power District, Missouri River Energy Services and Montana-Dakota Utilities Co.

The South Dakota Board of Minerals and Environment unanimously approved Big Stone II's air quality permits in November, but the U.S. Environmental Protection Agency filed objections in January citing deficiencies that needed to be addressed. South Dakota officials issued a revised permit, which received tacit approval in June when the EPA made no further objections.